Prosthesis for the replacement of hip joints are well known. Originally, only the ball-end on the head of the femur could be replaced but it has since proved possible to replace either part of the hip joint i.e., the acetabular socket of the joint or the ball-end on the femur.
Known acetabular cup implants, which form the socket portion of an artificial hip joint, comprise a metal cup shell, which is secured within the acetabulum in the pelvic bone of a patient, and an inner liner of plastic material which provides a spherical bearing surface for receiving the ball portion of the joint. The metal cup shell may be provided with an external thread to facilitate anchorage to the pelvic bone or may be secured by other means such as cement or screws. The plastic insert may be secured within the metal cup shell by numerous methods e.g. a retaining ring, press-fitting or force-fitting portions of the liner into apertures within the shell and thermally fitting the liner.
Many of the known methods for securing the plastic insert within the metal cup shell suffer from the disadvantage that there are a limited number of rotational positions in which the insert may be placed. This can be a serious disadvantage in cases where the insert has an angled face and it may not be possible to achieve the required alignment of the bearing surface of the insert relative to the ball portion of the joint without movement of the metal cup shell. Adjustments of the metal cup shell are often impracticable, particularly when the shell is secured by cement or screws.
Thermal fitting of the plastic insert relies upon the expansion of the insert at the temperature within the body to generate a high frictional force between the surface of the insert and the surface defining bore of the metal cup shell. In practice the insert is dimensioned to allow insertion into the bore of the metal cup shell at room temperature or lower temperatures and after insertion the plastic material will expand at the body temperature (37.degree. C.) such that the insert is a pressure fit within the bore. Unwanted movement of the insert is prevented by the frictional torque differential between the contacting surfaces of the metal cup shell and insert. While the thermal fitting of inserts has the advantage of allowing unlimited rotational position possibilities when fitting the insert thereby ensuring accurate alignment, the strength of the thermal fit has been questioned, particularly in the longer term, where creep of the plastic material can weaken the force between the contacting surfaces of the insert and metal cup shell. Severe weakening will allow rotational and/or distractional movement of the insert relative to the metal cup shell.